4.materi 4 - Teknik Multiple Akses
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Materi : 04
SISTEM KOMUNIKASI BERGERAK
Teknik Multiple Access pada Sistem Cellular JURUSAN TEKNIK ELEKTRO POLITEKNIK NEGERI MALANG MALANG, 2019
SISTEM SELULER :04
Definisi Multiple Access Adalah suatu teknik yang memungkinkan suatu titik ( Base Station ) untuk dapat diakses oleh beberapa titik yang saling berjauhan ( Subriber Station ) dengan tidak saling mengganggu Fungsi Multiple Access: •
Digunakan untuk mengorganisasi user dalam
•
memberikan komunikasi yang bebas interferensi Menyalurkan beberapa informasi secara serentak dalam satu spektrum. PT3163-SISKOMBER-MODUL:04
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Simplex Communication •
Normally, on a channel, a station can transmit only in one way. •
•
This is called simplex transmision
To enable two-way communication (called full-duplex communication) • •
We can use Frequency Division Multiplexing We can use Time Division Multiplexing
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Duplex Dupl ex Co Comm mmun unica icati tion on - FD FDD D •
FDD ( Frequency Division Duplex ).
Mobile Terminal M
Forward Channel Reverse Channel
Base Station B
Forward Channel and Reverse Channel C hannel use different frequency bands
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Dupl Duplex exiing - FDD •
A duplex channel consists of two simplex channel with different carrier frequencies •
F orward band band : carries traffic from
base to mobile •
F M
B Base Station
R
Mobile Station
R evers evers e ba band : carries traffic from mobile to base
Reverse Channel
Forward Channel f c,,F
f c,R Frequency separation
Frequency separation should be carefully decided Frequency separation is constant PT3163-SISKOMBER-MODUL:04
frequency
Duplex Dupl exiing - TDD •
A single radio channel (carrier frequency) is shared in time in a deterministic manner. • •
•
M
B
The time is slotted with fixed slot length (sec) Some slots are used for forward channel (traffic from base to mobile)
Mobile Station
Base Station
Some slots are used for reverse channel (traffic from mobile to base)
Slot number
0
1
2
3
4
5
channel
F
R
F
R
F
R
Reverse Channel
6
F
7
…
R
….
Forward Channel Ti+1
Ti Time separation
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time
Dupl ex Co Comm mmun unica icati tion on - TD TDD D Duplex •
TDD ( Time Division Duplex ).
Mobile Terminal M
M
B
M
B
M
B
Base Station B
A singe frequency channel is used. The channel is divided into time slots. Mobile station and base station transmits on the time slots alternately.
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Perbedaan FDD & TDD
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Frequency Division Multiple Access Principles •
A voice channel channel uses the same frequency band all the time –
•
Other users must use other frequencies
One frequency band is reserved for signali signaling ng F CH 1
F1
CH 2
F2
CH 3
F3 .
BW
. .
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Capacity of FDMA Systems Frequency spectrum allocated for FDMA system
…
Guard Band
channel
N
Bt
2 B gua guard rd
Guard Band
Bc
B : Total spectrum allocation Btguard: Guard band allocated at the edge of the spectrum band Bc : Bandwidth of a channel AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416. PT3163-SISKOMBER-MODUL:04
Time Division Multiple Access Principles •
•
A voice channel channel shares the same frequency band with other voice channels – Each channel gets assigned to a periodic time slot Signaling still use a dedicated frequency band F .
.
.
.
.
.
BW
T1 T2 T3
t
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TDMA Concept Downstream Traf Traffic: fic: Forward Channels: (from base to mobiles)
1
2
3
…
N
1
2
3
….
N
…
N
…
Logical forward channel to a mobile
Base station broadcasts to mobiles on each slot
Upstream Traf Traffic: fic: Reverse Channels: (from mobile to base) 1
2
3
…
N
1
2
3
….
Logical reverse channel from a mobile
A mobile transmits to the base station in its allocated slot Upstream and downstream traffic uses of the two different carrier frequencies. PT3163-SISKOMBER-MODUL:04
General Frame and Time Slot Structure in TDMA Systems One TDMA Frame Preamble
Slot 1
Guard Bits
Sync Bits
Information
Slot 2
Control Bits
Slot 3
Trail Bits
…
Information One TDMA Slot A Frame repeats in time
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Slot N
CRC
A TDMA Frame •
Preamble contains address and synchronization info to identify base station stat ion and mobiles mobiles to each each other other
•
Guard tim Guard times es are are used used to allow allow synchronization of the receivers between different slots and frames •
Different may have different propagation delays to mobiles a base station because of different distances.
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Efficiency of a Frame/TDMA-System •
Each frame contains overhead bits and data bits. •
Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits. efficiency bT
f
bOH (1 b ) x100%
T
T f x xR R
bT: total number of bits in a frame Tf : frame frame duration duration (seconds) (seconds) bOH: number of overhead bits Number of channels in a TDMA cell:
N
m
( Btot 2 B guar guard d )
Bc
m: maximum number of TDMA T DMA users users supported in a radio channel PT3163-SISKOMBER-MODUL:04
TDMA •
TDMA Efficiency – – –
GSM: 30% overhead DECT: 30% overhead IS-54: 20% overhead.
TDMA is usually combined with FDMA –
–
Neighboring cells be allocated allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other. There may be more than one carrier frequency (radio channel) allocated and used inside each cell. cell. Each carrier frequency (radio channel) may be using TDMA to t o further multiplex more user (i.e. having TDMA logical channels inside radio channels)
–
For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA. PT3163-SISKOMBER-MODUL:04
Contemporary TDMA Systems GSM
IS-54
PDC
DECT
(Europa)
(USA)
(Japan)
(European Cordless)
Bit Rate
270.8 Kbps
48.6 Kbps
42 Kbps
1.152 Mbps
Bandwidth
200 KHz
30 KHz
25 KHz
1.728 MHz
Time Slot
0.577 ms
6.7 ms
6.7 ms
0.417 ms
Upstream slots
8
3
3
12
Duplexing
FDD
FDD
FDD
TDD
Efficiency of Time Slots
73 %
80 %
80 %
67 %
Modulation
GMSK
p/4
GMSK
Adaptive Equalized
Mandatory
per frame
p/4
DQPSK
Mandatory
DQPSK
Optional
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None
Features of TDMA – –
Enables the sharing of a single radio channel among N users Requires high data-rate per radio radio chann channel el to suppo support rt N users simultaneously. •
–
High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread s parameter)
Transmission Transmissio n occurs in bursts (not continues) – –
–
Requires synchronization of the receivers. –
–
Enables power saving by going to sleep modes in unrelated slots Discontinues transmission also enables mobile assisted handoff Need guard bits, sync bits.
large
overhead per slot. slot.
Allocation of slots slots to mobile users should not be uniform. – –
It may depend on the traffic requirement of mobiles. This brings extra flexibility and efficiency compared to FDMA systems.
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Capacity of TDMA Systems •
Capacity can be expressed as –
System Capacity (the capacity of the ove overall rall system covering a region) –
–
Depends on: » Range of cells » Wheth Whether er the syste system m can can suppo support rt macro-cells macro-cells,, mi microcro-cells cells or pico pico-cell -cells. s.
Cell Capacity –
Depends on the radio link performance between a base-sation and mobiles: (carrier-to-interence) nce) ratio the system can operate for » The lowest C/I (carrier-to-intere example quality of transmission. This in turn t urn depends on the speech coding technique, desired speech quality, etc. ata-rate over the channel chan nelchannel which which depends depend s modulation efficiency » D (bits_per_second/Hz) and bandwidth. bandwid th. » The frequency re-use factor
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System Capacity
Cluster: 7 cells constitute a cluster. cluster. Cluster size = 7
y A
B
C B
D
x
G
A G
E
z A
F Frequency reuse factor is 1/7: same frequency is used every 7 cell. A is one set of frequencies, B is an other other,, etc. A mobile in cell x receives carrier signal from base x and interferences from base stations stations at cells y and and z. The c carrie arrierr signal signal streng strength th of all com combine bined d signal strength stren gth from inter interferin fering g base stations is called C/I or S/I ratio. ratio. PT3163-SISKOMBER-MODUL:04
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Comparing Systems –
AMPS Parameters – – – –
GSM Parameters
–
Channel bandwidth = 30Khz Required C/I: 18 dB Frequency re-use factor: 1/7 (cluster size=7) Required bandwidth per user = 30kHz.
Channel banwidth: 200 KHz Required bandwidth per user = 200/8 = 25 Khz. Required minimum C/I: 9dB Frequency re-use factor: 1/3 (cluster size=3)
– – – –
1
system_cap acity
channel_bandw id idth_ th_re required_per _user
1
system_cap acity
frequency_ reuse_fact or
cluster_si ze 1/ 3
capacity(GSM SM))
capacity(AMPS)
1/ 7
1 / 25
1 / 30
2.8
assuming other factorsare same like the range of base stations.
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Code Division Multiple Access Principles • •
•
Many voice channels share the same frequency band No timeslots; all channel uses the same frequency fr equency simultaneously all the time – Channels are differentiated by its code Signaling uses a dedicated frequency band Code
C3 C2 C1
F
t
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Code Divisi Code Division on Mult Multipl iple e Access Access (CD (CDMA) MA) •
•
In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term.
• •
Message consists of symbols Has symbol period and hence, symbol rate • Spreading signal (code) consists of chips • • •
Has Chip period and and hence, chip rate Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence) PN sequence is called a codeword
• • •
• •
Each user has own cordword Codewords areits orthogonal. (low autocorrelation) Chip rate is oder of magnitude larger than the symbol rate.
The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code The sent signal is recovered by despreading process at the receiver.
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CDMA Advantages •
Low power spectral density. • •
•
Interference limited operation •
•
•
All frequency spectrum is used
Privacy •
•
Signal is spread over a larger frequency band Other systems suffer less from the transmitter
The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit
Reduction of multipath affects by using a larger spectrum Random access possible •
Users can start their transmission at any time
• • • • • •
Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity Higher capacity than TDMA and FDMA No frequency management No equalizers needed No guard time needed Enables soft handoff
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Sprr ead spe Sp spectr um techno hnolo logy gy
Pseudo-random modulation
A Ant ntii -jam -jam li ty i nte nterr ce cep pt L ow pr obabi lity
PRM
hasil sprea spread di ng
jamm jam ming
PRD
informasi
Open sta standa ndarr d
C omm ommer cial ci al per per ta tam ma Septem tembe berr 1995 1995 : M otor tor ola
hasill de hasi despr spre ead adii ng
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M enggunaka Me nggunakan n codi ng Sattu r uang uang dengan ngan se sejj umla umlah h pasanga sangan n Sa U dar ara a se seb bagai agai medi a codi ng siste si stem m B ahasa adalah cod B aha hasa sa la laii n dia diang ngggap se seb bagai no noii se asang ngan an lai lain n dap dapat ber g ab abung ung ber sama sama sa sam mpai noise noi se tte er tentu tentu P asa Jii ka bi sik sik-b -bii sik makin ki n banya nyak pasanga sangan n yang dapat di tampung ung J
The CDMA Cocktail Party
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Prinsip CDMA A dal dalah ah s eju ejuml mla ah us er meng g un una ak an r es our ourcc e band R F yyang ang s ama nam na mun s et etiap iap us user er dibeda dibedakk an deng an me meng ng g una unakk an kode-kode orthogonal. S ta tandar ndar IIS S -95, la lajj u da data ta pada ak hi hirr s pr prea eadi ding ng ada adala lah h 1,2288Mc 1,2288Mcps ps dan da n iini ni me memb mbutuhkan utuhkan ba bandwidth ndwidth llebih ebih kkur ura ang 1,25 MHz. K inerja s is tem C DMA diba dibattas i ol oleh eh inte interferens rferensi,i, a artinya rtinya k apa pass ita itass dan da n k ua ualli ta tass dibat dibata as i ole oleh h daya daya in intterfer erferens ens i yya ang te terr jadi pa pada da b ba and R F ya yang ng dipa dipakai. K apa pass ita itass didifini didifiniss ik an s eb eba ag ai jjum umllah us er s eca ecara ra ssimul imulttan ya yang ng dapa da patt d diduk idukung ung ol oleh eh ssis is tem em.. K ua uallita itass ada dallah B E R yang dipers yara yarattk an da dallam m mel ela aya yani ni us er
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sprr eader sp
desspr de pread eader er
identik C od ode e gener nerat ator or •
•
bandw ndwii dth gener nera ato torr
C od ode e g ener nerat ator or
bandw ndwii dth i nfor nf orm masi
P r ose sess de desp sprr eadi ng akan meng ngha hasi silk lka an outp utput ut hanya j i ka gene generr ato tor r disisi penerima berkorelasi dengan signal yang akan didespreading untuk uk si sigg na nall ya yang ng la laii n. dan ber sif si fat ortogonal unt
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C ont nto oh pe penga ngattur ura an PI P I L OT OT_PN _PN
a 19
a 5 a a 20
28 a 14
6
a 27 a
a 21
13 a a 36
7 a 30
a 22
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E xcelle cellent nt ca call ll quali quality ty
Soft So ft hand hando off P ath di di ve verr si sitty ( R ake r ecei cei ve verr )
Ur ban a arr ea
BTS
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Balancing
kualitas
kapasitas coverage
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C(t)
C(t-2)
delay adj.
korelator
B TS
C(t-n)
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P ower kontr ntro ol sa sang nga at ba bag us
Subc-2 Sub c-2 Subc-1 Sub c-1 10 km
5 km Subc-1 Sub c-1 Subc-2 Sub c-2
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Sho Shop phisti histi cated V OC OCOD ODEE R
PCM
01001110101110010110 (64KBPS)
wireline
VOCODER
010011101(8,6KBPS)
wireless
Ad dvanc nce e E r r or A
de detecti on & C or r ecti on : K onv nvo olu lusi-V si-Vii ter bi
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K apasit si tas ttii ng ngggi Sect Se cto or -A
Sect Se cto or -C
Sect Se cto or -B
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C ell b brr eathi hing ng ( 1)
pilot
traffik
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C ell b brr eathi hing ng ( 2)
?
?
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CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1
One bit period (symbol period)
Data
PN-Code (codeword)
1
1
0 1
1
1
0
1
0
1
1 1
1
1
0
1
0
1
1
Coded Signal
Input to the modulator modulator (phase (phase modulation) modulation)
Chip period
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Processing Gain Main parameter of CDMA is the processing gain that is defined as:
•
G p
B spr spread ead
R
Bchip
R
Gp: processing gain Bspread: PN code rate Bchip: Chip rate R: Data rate •
IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. –
1.228 Mhz chipping rate
–
1.25 MHz spread bandwidth
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Near Far Problem and Power Control •
At a receiver, thesources). signals may come from various (multiple •
•
•
•
•
The strongest signal usually captures the modulator. The other signals are considered as noise Each source may have different distances to the base station
In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. •
B
Therefore the receiver power at the base station for all mobile users should be close to each other. This requires power control at the mobiles.
Power Control: Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.
pr(M)
M M
M M
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DSSS Transmitter Message m(t)
sss(t)
Baseband BPF
+
Transmitted Signal
p(t) PN Code Generator
Oscillator f c
Chip Clock
s ss (t )
2 E s T s
m(t ) p (t ) cos( 2p f c t )
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DSSS Receiver s1 ( t )
IF Wideband Filter s
t
() Received DSSS Signal at IF ss
Phase Shift Keying Demodulator
p (t )
PN Code Generator
s1 (t )
2 E s
T s
Synchronization System
m( t ) cos( 2p f c t )
m(t )
Received Data
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Spectra of Received Signal Spectral Density
Interference
Spectral Density
Signal
Interference
Signal
Frequency Frequency
Output of Wideband filter
Output of Correlator after dispreading, Input to Demodulator
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CDMA Example (*) R
Data=1011…
Receiver (a base station)
Data=0010…
A
B
Transmitter (a mobile) Codeword=010011
Transmitter Codeword=101010
Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading. (*) This example is adapted from the CDMA example example of Prof. Randy Katz at UC-Berkeley. UC-Berkeley.
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CDMA Example
Data Code
transmission from two sources
1
A Data A Codeword
–
0
1
1
0 1
0
0
1
1
0 1
0
0
1
1
0
1
0
0
1
1
1 0
1
1
0
0
0
0
0 1
1
1
0
1
1
0
0 1
1
0
1
0
0
0
1
1
1
0
1
0
A Signal
0
B Data
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0 1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1 1
0
1
0
1
0
B Codeword Data Code
B Signal Transmitted A+B Signal
0
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CDMA Example
–
recovering signal A at the receiver
A+B Signal received A Codeword at receiver (A B) Code
Integrator Output
Comparator Output
0
1
0
Take the inverse of this to obtain A
1
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CDMA Example
–
recovering signal B at the receiver
A+B Signal received B Codeword at receiver
(A B) Code
Integrator Output
Comparator Output
1
1
0
Take the inverse of this to obtain B
1
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CDMA Example
–
using wrong codeword at the receiver
A+B Signal received Wrong Codeword Used at receiver
Integrator Output
Comparator Output
X Noise
0
1
1
codeword will not be able to decode the original data! Wrong codeword
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Cellular Systems and MAC Cellular System
Multiple Access Technique
AMPS
FDMA/FDD
GSM
TDMA/FDD
USDC (IS-54 and IS-136)
TDMA/FDD
PDC
TDMA/FDD
CT2 Cordless Phone
FDMA/TDD
DECT Cordless Phone
FDMA/TDD
US IS-95
CDMA/FDD
W-CDMA
CDMA/FDD CDMA/TDD
cdma2000
CDMA/FDD CDMA/TDD
View more...
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